1. Field of the Invention
The present invention relates to a fundus camera which performs alignment while observing an operation distance index.
2. Description of the Related Art
In a conventional fundus camera, when an operator photographs a fundus image, a light flux is projected from the periphery of a pupil of a subject's eye, and a fundus reflection light flux is taken out from a center of the pupil as a photographing light, so that a photographing light flux has a small diameter. Thus, if the light flux deviates even a little, the pupil shades the light flux, so that a flare easily enters in an imaging plane. For performing correct positioning, it is necessary to stabilize a fixation, match a light axis of the subject's eye and an optical axis of a fundus camera optical system, and adjust alignment to a proper position.
Japanese Patent Application Laid-Open No. 62-34530 discusses a fundus camera which projects an alignment index on a cornea of a subject's eye, and determines whether the alignment index is on a proper alignment position in a state that an alignment image which is a reflection image of the projected alignment index in focus. Japanese Patent Application Laid-Open No. 7-31590 discusses a fundus camera which projects an alignment index on a cornea of a subject's eye, and determines whether the alignment index is on a proper alignment position based on separation or matching of an alignment image.
Japanese Patent Application Laid-Open No. 7-31590 and Japanese Patent Application Laid-Open No. 2000-287934 discuss a technique for moving a light guide for projecting an operation distance index. In this technique, the light guide is moved so that a proper operation distance when a fundus peripheral part is photographed is becomes longer than the proper operation distance when a fundus center part is photographed. However, this operation is not for detecting the operation distance.
In the above described conventional fundus cameras, since a light emitting diode (LED) light source having one kind of single wavelength is used as a light source for an alignment index, it can be determined whether the alignment index is in a proper alignment position or not. However, it cannot be determined whether the fundus camera is in a near position with respect to a subject's eye or in a far position in the proper alignment position.
Therefore, for operating the fundus camera to a proper alignment position, an operator moves the fundus camera once toward either direction of closing to or separating from the subject's eye, and determines a direction for closing to the proper alignment position based on whether a defocusing degree and a separation degree increase or not. There is no problem when an operator can move the fundus camera to a right direction for alignment by chance. However, when the operator moves the fundus camera toward an inverse direction, the operation for moving the camera is wasteful.